As, for example, computer networks have spread, and the performance of computer systems has been improved, access to a huge amount of electronic document data (hereinafter called document data) has been facilitated. Document data search enables location of necessary document data in a huge amount of document data. Document data search is performed on various types of objects. For example, in the case of search engines in Internet browsers, Web pages in servers or intermediate servers (also called proxy servers) distributed worldwide are searched. Moreover, in companies, document data accumulated in business operations is searched. Typical methods for searching document data include a method for finding a word or a phrase in a co-occurrence correlation with a search keyword by analyzing the correlation between the search keyword and words or phrases in document data. For example, document data highly relevant to a word “IBM®” (registered trademark of IBM Corporation) can be appropriately found from objects to be searched, by finding a word or phrase having a strong correlation with the word “IBM®” (registered trademark of IBM Corporation), on the basis of the word or phrase. A method for searching document data is disclosed in, for example, Christopher D. Manning, et al., “An Introduction to Information Retrieval”, Chapters 6, 14, 16 and 18, Cambridge University Press, Apr. 1, 2009.